Saussure's Course in General Linguistics was compiled from his lecture notes and published after his death. The first thing to know about his concept of the structure of language is the arbitrary nature of the sign. That is, given that there have been thousands of human languages (written and spoken), he questioned how and why there can be a thousand different words for the same thing. He concluded that there is no essential or inherent relationship between a word (signifier) and its meaning/referent (signified). In other words, the word "tree" or the German "baum" have been accepted as signifiers of tree for cultural reasons. There is nothing meaningful about the sound of the words or the way the letters are written which necessitates their relationship to the meaning or actual tree.

In other words, theoretically, any word can be a signifier for tree. Cases of onomatopoeia like "buzz" are exceptions but they are also arbitrary because they are determined by our audible sense of a buzz and how we can vocally articulate the sound as a word; therefore, the relationship between the sound of a buzz and the word "buzz" is limited to our senses. There is not an inherent connection between the word and the sound on any theoretical or meaningful level.

The signifier "tree" means tree - not because people associate it with the idea or thing tree- but because a person/persons links syntagmatic and associative relations. Each word/signifier in the sentence (The tree has yellow and red leaves in autumn) is a syntagmatic element. The way they interact and more specifically, they way they differfrom each other is what makes meaning of each (and the whole) possible. A person/persons will also link associative signifiers such as "season," "color," "life," and any other signifiers that can, in some way, be associated or differentiated from the syntagmatic signifiers. Therefore, a signifier expresses meaning as a result of its differences between and within a chain of other signifiers. A signifier (word) on its own has no meaning. If an alien asks you what "tree" means, you will have to describe other signifiers ("trunk," "forest," "plant," "leaves," etc.)

Saussure also noted the difference between la langue (the general term for language which includes the codes and structures that emerge as different languages) and parole (the individual utterances which enact language). He uses the analogy of chess. La langue is the rules of chess and parole is each move. Saussure noted that one could study the pieces on the board at a particular time (synchronic) or one could study the development of the game and other games over time (diachronic). Thus, for Saussure, the way a language (game) changes has more to do with the rules (he uses the word "conventions") than it does with individual utterances (moves). Therefore, Saussure focused more on the structure of language (la langue) and he focused more on language at a particular time (synchronic).

His concept of the structure of language is based upon the arbitrary nature of the sign. Each signifier expresses a meaning (a signified) by its relation/difference to other signifiers. The way that the signifiers interact/differ is primarily determined by the structure (grammar, oppositions, associations, syntax, metaphor, etc.) of a language. The bottom line is that words have meaning because of difference.